1. Field of the Invention
The present invention is directed to the field of temperature responsive devices. More particularly still, the present invention is directed to that portion of the above-noted field which is concerned with the sensing of relatively high temperatures in the range of from about 700.degree. F. to about 1500.degree. F. More particularly still, the present invention is directed to that portion of the above-noted field which is concerned with generating an electrically measurable response to changes in sensed, relatively high, temperature. More particularly still, the present invention is directed to that portion of the above noted field which is concerned with the provision of ceramic semiconductive elements having an electrical parameter which varies predictably and repeatably in response to variation in a sensed temperature. More particularly still, the present invention is directed to that portion of the above-noted field which is concerned with the provision of a ceramic semiconductive element having a resistance which varies predictably and repeatably in response to variation in a sensed temperature. More particularly still, the present invention is directed to that portion of the above-noted field which is concerned with the provision of gas temperature sensing elements suitable for insertion within the exhaust system of a combustion source in order to monitor the temperature of the exhaust gas within the system.
2. Description of the Prior Art
It has been determined that the operation of a conventional automotive internal combustion engine produces substantially quantities of deleterious gaseous combustion by products. The principal pollutants so produced are hydrocarbons, carbon monoxide and varies oxides of nitrogen. Extensive investigation into the combustion process, examination of alternative combustion processes and detailed studies of exhaust gas treatment devices have led to the conclusion that the use of a catalytic converter within the exhaust system of an internal combustion engine provides a practical and effective technique for substantially reducing the emission of the deleterious gaseous combustion by products into the atmosphere.
A catalytic exhaust gas treatment device or converter which is capable of substantially simultaneously converting all three of the aforementioned principal pollutants into water, carbon dioxide and gaseous nitrogen is referred to as a "three-way catalyst". However, for the known three-way catalyst devices to be most effective, the gaseous by products introduced into the converter must be the by products of combustion of a substantially stoichiometric air/fuel ratio. Such three-way catalyst devices are said to have a very narrow "window" of air/fuel ratios at which the device is most efficiently operative on the three principal pollutants. By way of example, .lambda. is the air/fuel ratio normalized to stoichiometry, the window may extend from about 0.99.lambda. to about 1.01.lambda.. Such a three-way catalyst device is described, for example, in U.S. Pat. No. 3,895,093 issued to Weidenbach et al. on July 15, 1975, assigned to Kali Chemie Aktiengesellschaft and titled "Catalytic Removal of Carbon Monoxide Unburned Hydrocarbon and Nitrogen Oxide from Automotive Exhaust Gas". For air/fuel ratios of the combustion mixture on either side of the window, one or two of the principal pollutants will be converted in only very small percentage efficiencies. Within the window, the three principal pollutants will be converted at very high efficiencies approaching 90% in some cases. In view of the narrowness of the catalytic converter window it has been determined that the associated internal combustion engine must be operated with a combustible mixture as close as possible to stoichiometry.
The most satisfactory technique for assuring continuous or substantially continuous operation at the optimum air/fuel ratio is through the utilization of an appropriate feedback mechanism. In implementing suitable feedback control systems it has been proposed to employ sensors responsive to the chemistry of the exhaust gases, that is, the hot gaseous combustion by-products, in order to control the precise air content and/or fuel content of the air/fuel mixture being provided to the engine. One type of electrochemical exhaust gas sensor employs a ceramic material which demonstrates a predictable electrical resistance change when the partial pressure of oxygen of its environment changes. An example of such a material is titania (titanium dioxide having a general formula TiO.sub.2). Such sensors can be fabricated generally in accordance with the teachings of U.S. Pat. No. 3,886,785 issued to Stadler et al., titled "Gas Sensor and Method of Manufacture" and assigned to the assignee hereof. Tests of such devices have shown that at elevated and substantially constant temperatures the devices will demonstrate a virtual step change in resistance for richto-lean and lean-to-rich excursions of the air/fuel ratio of the combustion mixture producing the exhaust gas environment of the device.
A principal difficulty which has been encountered with such variable resistance devices resides in the fact that such devices will demonstrate a measurable resistance change which is also a function of change of the temperature of the ceramic material. For example, a temperature change of about 500.degree. F. produces a measurable resistance change on the order of magnitude of a sensed rich-to-lean or lean-to-rich air/fuel mixture change. Such a temperature variation may be encountered, depending of course to some extent on the location of placement of the sensor within an exhaust system, during acceleration of the associated engine from idle speed to highway speeds. Heretofore, exhaust gas sensors which employed a variable resistance sensor ceramic have required that the temperature of the material be relatively closely controlled for reliable use in a feedback system intended to provide an internal combustion engine with very precise air/fuel ratio control.
Temperature control of the associated sensor has proved to be an expensive and not wholly satisfactory technique for adapting such variable resistance sensor ceramic materials to air/fuel ratio feedback control systems in automotive internal combustion engines. In order to overcome the difficulties encountered, it has been suggested to combine a thermistor in an electrical series circuit with the variable resistance exhaust gas sensor and to operate the electrical series circuit as a voltage divider network deriving a useful voltage signal from the junction between the thermistor and the variable resistive gas sensor element. However, known thermistors are temperature limited to such a degree that it is an object of the present invention to provide a thermistor capable of operating within the exhaust gas environment of an internal combustion engine. More particularly still, it is an object of the present invention to provide a semiconductive variably resistive ceramic element capable of withstanding temperatures in the range of from about 700.degree. F. to about 1500.degree. F. for extended periods of time and which is otherwise capable of tolerating the adverse environment of an automotive engine exhaust system. It is a further and specific object of the present invention to provide a sturdy, ceramic, high temperature thermistor which is electrically compatible with use as a portion of a voltage divider network having as the other portion thereof a variable resistive oxygen responsive ceramic member.